1. Field of the Invention
The present invention relates to thermal image transfer systems, and to donor sheets useful in such systems, and to a process for thermally transferring images.
2. Background of the Art
Many imaging systems have been developed to be used with computer generated and other electronically generated images. This development has been necessitated by the generation or transmission of electronic images and the need for hard copy prints of such images, both in black and white and color. Originally silver halide imaging systems were used for such image generation, and such systems still can provide high quality images. In certain areas of the market, lower image quality can be tolerated and lower costs are essential. Ink-jet printing and thermal dye transfer systems have found increasing acceptance in these markets.
Ink jet printing has suffered in its acceptance because of a number of technical problems, not the least of which is a tendency of print heads to clog. This requires an intolerable level of maintenance and a complete shut down of the system during servicing. Furthermore, image colors tend to be unstable and color gradation has been virtually non-existant. Color gradation has been quite limited in commercial thermal colorant transfer systems, although significant improvements in these problems have been made.
The technology of thermal colorant systems can be divided into two fields, mass transfer and dye sublimation transfer. The term mass transfer is used to refer to systems in which both the colorant and its binder are transferred from a donor sheet to a receptor sheet (or intermediate carrier sheet). Because of the relatively large size of the transferred material, (a particle comprising both colorant and binder), color gradation or continuous tones in the image is difficult to achieve. Furthermore, if the colorant is a dye it exhibits more limited aging stability than do pigments.
The term sublimation transfer is used to refer to systems in which essentially only the colorant is transferred by sublimation or vaporization to a receptor sheet. This type of process leaves behind in the donor sheet any binder which might have been used in the donor sheet.
In the mass transfer technology area such improvement has been made in the design and thermal control of the print head. A good example of this approach is is given by S. Merino of Matsushita Electric Company, Ltd. in a paper presented at the August '86 SPSE Conference on Non-Impact Printing Technologies in San Francisco. He described "thermo-convergent ink transfer printing (TCIP) as a system in which the shape of the heating elements of the print head are optimized and the energy pulses are controlled so that half-tone or approximately continuous tone reproduction is much improved when wax-colorant donor sheets are used. Understandably the donor sheet has been the target of improvement work in recent years. Japanese Kokai, J59-224394 discloses the use of two imcompatable binders in which the dye is dissolved. This results in the mass transfer of relatively small particles of colorant. Combining this donor sheet with good print head control is reported to give some low level of color gradation.
European Patent, EPO 163297 teaches the use of high melting-point particles with diameters larger than the thickness of the ink layer which particles serve as heat conductors to aid in the transfer of the color mass.
A paper entitled "Thermal Ink Transfer Sheets for Gradated Print" by Tagushi et al, of Matsushita given at the SPSE Conference in San Francisco on Aug. 24-26, 1986 briefly described a system claimed to yield improved mass transfer quality. This system makes use of one resin and colorant in the donor sheet and a different resin in the receptor sheet. The modulated thermal signal in the print-head causes changes in the "melt, compatibility, adhesion and transfer between the two resins" thus producing a continually graduated print.
Japanese Patent JP 62-292483 discloses a thermal transfer sheet having a thermal transfer layer which comprises a mixed wax of at least two components and a colorant. The layer comprises at least 60% by weight of the combination of waxes having a melting point in the range of 45.degree.-70.degree. C. and another thermal melting material having a softening point within the range of 100.degree.-200.degree. C. Neither of these materials are shown to include chlorinated waxes. However, an optional third ingredient, other waxes that may be mixed with A and B, includes amongst the more than thirty alternatives "chlorinated paraffin wax" (page 8 of translation). The system therefore requires that at least 60% of all thermally softenable materials be other than the chlorinated wax alternative.
Japanese Patent, JP 58-162678 discusses an ink containing chlorinated paraffin wax, and is used for a thermal transfer ink which is coated on paper. The use of the chlorinated paraffin wax is noted for improved shelf-life characteristics for the thermal transfer coating. There is no mention of improved transparency or clarity of colors.
U.S. Pat. No. 4,503,095 and U.S. Pat. No. 4,572,684 discuss a thermal transfer ribbon composition that contains a coloring agent and a hot-melt vehicle for a thermal transfer composition. These patents disclose that the coloring agent and the hot-melt vehicle used in each ink layer preferably should have refractive indexes which are near to each other. These patents do not mention the use of chlorinated paraffin waxes.
U.S. Pat. No. 3,736,133 discusses a method of forming ink absorbent transparencies comprising applying a lacquer to a polymeric film transparency, said lacquer comprising a substantially transparent resinous binder pigmented with an ink absorptive pigment exhibiting substantially the same refractive index as that of the binder, and drying said lacquer on said transparency. The pigment contains an ink absorptive pigment with high effective surface area which has a refractive index closely matching that of the binder in which it is to be used. Pigments specified match a certain range of near refractive index qualified resins, but they must also have the property of having an exceptionally high absorptive power for inks.
U.S. Pat. No. 3,601,484, Dybvig, et. al., discloses that configurations for carrier or donor sheet size can be in exact line up with the receptor sheet size. Also, Great Britain patents 1,278,325, 1,281,859, and 1,281,860 clearly detail elongate web material coated in sequential color arrangements, and each color zone being of equal size to the color separation image to be reproduced. This configuration is discussed in more exact size arrangement in U.S. Pat. No. 4,503,095,